Automatic plotter



March 7, 1950 R. w. BENFER AUTOMATIC PLOTTER Filed March so, 1945 2 Sheets-Sheet l /NVENTOR R. W @ENFER March 7, 1950 R. w. BENFER 2,499,941

AUTOMATLC PLOTTER Filed March 30, 1945 2 Sheets-Sheet 2 FIG. 3

/NVE/VTOR R. W BENFER ATTORNEY Patented Mar. 7, 1950 AUTOMATIC PLTTER Richard W. Benfer, Morristown, N. J., .assigner to Bell Telephone Laboratories, incorporated, New York, N. Y., a corporationof New York Application March 30, 1945, Serial No. 585,697

10 Claims.

This invention relates to anautomatic plotter for translating electrical impulses into signal indications and displaying the same at predetermined circumferential and radial positions on a plotting surface of large diameter; and more particularly to an automatic .plotter employed .as an extended scale plan .position .indicator in an object location system.

It has heretofore been .customary in connection With object location systems to provide at an information center a `plotting board carrying a grid map of a large .exploratory area that is being scanned and to transmit` telephonically by spoken word to the operators stationed at the plotting board, to be manually marked on the board, target signals which appear .upon ya. small oscilloscopio plan position indicator, or upon a plurality of Oscilloscopes eachoneof which is receiving pulseecho signals froma portion of the Whole eXplorn atcry area. Such anarrangement is satisfactory if the targets simultaneously under observation are relatively few. But where there are many targets simultaneously under observation in the exploratory area, the spoken telephonic transmission of essential data to the operators. at the information center gives rise to confusion, inaccuracy and delay.

It is the particular .object of this invention to provide an extended scale plotting vdevice at the information center for automatically translating into visual indications on or with reference to the grid map of the area .under observation a period .ically repeated sequence oi electrical impulses the time interval between the impulses of which determines the distribution `over the grid map of the .plotter of luminous signals each of which corresponds in azimuth and range on the `grid .map with the location in azimuth and range of the corresponding object in the exploratory area.

In its preferred embodiment the present invention `includes a plotting table of a sufciently large diameterso that it may, if desired, be attended by a number of operators. The plotting table is of transparent Imaterial preferably coated on its vunder-surface with va ,phosphorescent or photoluminescent material, and .carrying a large scale grid map of the exploratory area on its uppersurface. The phosphorescence or photoluminescence of the under-surface is preferably excited by ultraviolet light and has such adegree of persistenceas may 'ce desired. The admission of the exciting light to the proper point .on the photoluminescent coating is controlled by circumferential and radial scanning members which,

in the case of ya radar or object .location system,-

are in synchronism and phase in their movements with the azimuth and range scanning movements of the antenna and .its associated range scanning elements. The scanning members at the plotting table Aare operatively .substantially coextensive with the photoluminescent-surface .of the plotting table, and are supported in such a way that theoperating plane of rotation of vtheir scanning parts lies in close proximity tothe plane of the photoluminescent under-surface. Thus, the exciting signal-controlled light is substantially directly applied tothe particular point .on the surface determined by the relative positions of the scanning `elements as they are rotated beneath the table surface.

The` automatic vplotter'of the present invention is'capable of use in various environments Where it may be desired automatically to translate elec trical impulses into visual signals appropriately placed on an extended plotting surface; but as the plotter has been designed and is particularly adapted -for use in an object location system of the early-Warning radar type, it YWill be specifically disclosed and its operation described in connection with such a system. A system of the nature of that to be described is disclosed ina copending application of W. C'. Tinus, Serial No. 585.668, filed March 30, 1945.

The invention will .be more clearly understood by reference to the following detailed description, .taken in connection With the accompanying drawings forming apart thereof, in which:

Fig. l shows in schematic form an object'location system employing the preferred lform of plotter of the present invention;

Fig. 2 is a schematic diagram of a circuit which vmay be used for .converting the object location impulses into ultraviolet light at the plotter;

Fig. 3 shows the -plotting table in sectional ele- Vation;

Fig. i is a topplan View of the plotting table with parts broken .away to show the relation to each other of the slottedscanning elements: .and

Fig. 5 is a view showing in sectiona .part of the upper portion of the plotting table.

Referring more particularly to the drawings, an object location system including the plotter -cf the present invention is schematically represented in Fig. 1. Portions of the system which represent elements that are wellknown and commonly employed in the art are represented Ain block schematic form. In the arrangement of Fig, l, power is furnished by the source of power supply H to the pulse modulator l2, Which lis connected .by way of y.the -radioztransmitter .t3 :and

a TR box I4 to the .antenna Il. The pulse modulator I2 produces pulses at any suitable periodicity, such, for example, as 400 per second, each pulse having a length of from one or a fraction to ten microseconds. These pulses are modulated by a suitable ultra-high frequency carrier and are transmitted by way of the radio transmitter I3, TR box I4 and antenna Il. By way of example, the pulse modulator I2 can comprise an oscillator for providing a sine wave having a suitable periodicity. This oscillator energizes a pulse generator of any one of several suitable types well known to the art; for example, that disclosed in United States Patent 2,117,752, issued May 7, 1938, to L. R. Wrathall, which provides an energy pulse at a particular point of each cycle f the input wave supplied to it. The pulses from the pulse generator, modulated by the ultra-high frequency carrier, are applied by way of the TR box I4 to the antenna II, which serves both as a transmitting antenna and as a receiving antenna to receive waves reflected from one or more objects within the range of the transmitted pulses. The antenna II may be of any suitable type, for example, the polystyrene polyrod type' disclosed in an application of G. E. Mueller, Serial No. 469,284, led December 17, 1942, now Patent 2,425,336, issued August 12, 1947. The reflected waves picked up by the antenna I1 pass by way of the TR box I4 to the radio receiver I5.

The TR box I4, or transmit-receive box, may be of any desirable type, for example, the type employing a Western Electric Company 709A tube. This tube is essentially a gas discharge protective tube mounted in and forming part of an electrically resonant cavity in electrical communication With the interior of the coaxial transmission line leading to the transmitting and receiving antenna I'I. During reception of the low voltages of received energy the gas of the tube is not ionized, the cavity is tuned to electrical resonance with the signal, and the received energy passes through to the radio receiver. During the emission of a pulse from the transmitter the high voltage due to the pulse ionizes the gas in the tube, thus detuning the resonant cavity and preventing any substantial part of the energy of the pulse from reaching the radio receiver.

The reflection or echo waves, after amplification and detection in the radio receiver I5, are applied together with a sweep voltage from the sweep circuit I6 to the cathode-.ray or electron beam tube I9. In order that there may be discrimination against random disturbances of relatively large magnitude and in favor of the reflected target signals, the radio receiver I5 preferably includes a clipper or amplitude-limiting device, such as that disclosed in an application of D. Mitchell, Serial No. 464,271, led November 2, 1942, now Patent 2,395,575, issued February 26, 1946, for limiting the intensity of all voltage variations to that of the maximum signal desired. By way of example, the clipper will remove all voltage Variations of greater magnitude than that of the strongest echo.

The sweep voltage is a sweep wave of sawtooth form produced, for example, by a sweep circuit such as disclosed in Patent 2,178,464, issued October 31, 1939, to M. W. Baldwin, Jr. Pulses from the pulse generator and modulator I2 are communicated to sweep circuit I6 to initiate each of the sweep waves so that the electron beam in the cathode-ray tube I9 starts sweep cycles synchronously with the transmission of pulses. If desired, by the use of well-known variable delay 4 means, the sweep wave can be initiated a predetermined short interval after the transmission of each of the pulses to the antenna I'I.

The cathode-ray tube |19 is one of the elements of the cumulator, which together perform the function of receiving, distributing in order and storing the target echoes or received signals at the video frequency rate at which they are received from the scanning antenna, and subsequently reducing them to an audio frequency rate at which they are transmitted by audio frequency circuits to the distant plotting board 0r table. The cathode-ray or electron beam tube I9 may conveniently be of the general type of the tube 20 disclosed in an application of Gould and Mertz, Serial No. 505,028, led October 5, 1943. Similarly to the tube 20 of the above-mentioned application, the tube I9 of the present application comprises an evacuated container enclosing an extended series of collecting elements 20, an electron gun for generating, focussing and accelerat- Y ing a beam of electrons of such velocity as to be capable of building up negative charges on the collecting elements, and a pair of electrostatic delecting plates 2I for causing the beam of electrons to impinge upon each of the collecting elements 20 in turn. The electron beam generated by the electron gun is modulated by the video target signals or reflected impulses detected and amplified =by the radio receiver I5. The electron gun may comprise a cathode 22, a control elec trode 23 and accelerating anode members 24 and 25. The control electrode 23 is normally maintained at any suitable negative potential with respect to the potential of the cathode 22 by appropriate connection with the serially arranged resistances 26 connected across the terminals of the direct current power supply 21. The electron accelerating elements 24 and 25 are so connected with the resistance 26 as to give them appropriate positive potentials with respect to the cathode 22, the cathode being so connected as normally to have a positive potential with iespect to the control element 23. The potential applied to the various electrode members and the location and shape of these members are such that the beam of focussed electrons impinges successively upon the elements 20 as it is moved under the iniluence of the deiiection plates of the sweep circuit. These collecting elements 20 may, if desired, be treated with carbon so as to insure that the ratio of primary electrons striking each element to the number of secondary electrons leaving it is less than one.

Each of the elements 20 swept by the beam of electrons is connected through a parallel resistance member 28 and capacitance member 29 to ground and the positive pole of the direct current power supply 2l. The resistance 28 allows the charge imparted by the electron beam gradually to leak oi, but the time constant of each circuit is longer than the time between successive pulses projected by the antenna il. The charge accumulated at each of the elements 20 therefore is gradually built up by the series of received impulses from each target at the repetition rate of the transmitted pulses. Each of the collecting elements 2li of the cathode-ray tube with its storing capacitance 29 and shunted resistance 28 is connected with a corresponding contact member on a commutator switch 3D. These contact members are adapted to be successively engaged by a rotating switch arm 3l. As each is passed over, the charge accumulated in the associated condenser is transmitted to the assemblage of elements?t designated as? tiles. plotter' byWinry;,ofta'A cuittthat"isrschematieal1y7 representedE as passing tlfirougliV an' ampli'er"andl'ilter-v 3-2 and* their through anotlier amplifier 33a A't the plotting oenter' wherethe' information represented byV tl'ie-` stored: target'- signalst4 is to\ bel used; the signals areanipliiled4 by; tlieI amplifier' 335` modulated? in modulating oscillator 55'; and delivered`A through slip'1 ring and". associated? brush. connectionsr to an=u1tra.violetilamnfB'E 'The ultra-violet lamp:14 35imayfconveniently bewlflati'sy known asia] germicidal lamp.;I a: lamp which generally.` like:` they commercial fluorescent lamps:

except: tliatiA the tubular` Shelli isz. made? of' a type electrodesv wouldi be: off. the; equipotentiala heaterfV type; Theheating.'cument-issuppliedrbyfa source`- of: current which; is applied? by; Way of.. brushesi in.l

' engagement With'slip ringsf Ifa'nd52'v on the shaft;

carrying-thea rotating; platformf. 4 u? off the plotter;

The'I heating; laments4'9. andi. 501 magybeiseriallyl included ini this? circuit; asfin tlieicaseffofv conr-v meroial; fluorescent vlamps: l

The signal;impulsefissappliedvthroughthe:brush and-fslip. ring. 53itthefoleotrodef: 5.0i .corresponding to theslip:ringeandiassociated;brush;connectionsl 341- of; tneemoregs'chernati showingg-iriz Eig; 1:, .the-tl' energy2of-thi1np11lse"beingpreventedroimbeinga dissipated in; thea'lamentlieatings; cincuitz'by" the. interpositiom ofi4 high; frequeneyv choke; coilsi 514 in.: the: conductive :patlisf extending frornzlament: 5B: The signaliisrpreferably appliedfthrouglifthe medium of' a modulating 1 oscillator.v` 5?Y Wfl'iich:I may be part ofthe;unithdesignatedfas:335m7 Fig; 1; or may. bein' additionxthereto.- The-unit iiprei. erably' generates ai high.:frequenoywave;.say,i one*- megacycleg. Whichi is modulated; by. the:y signa-la. The arrangement: is' preferably such; that:l nora. 'mallythere iszveryflittle? it any. flow'ofw oscillatory high; frequenoyf" current.A through the lamp- 35. Each: incoming signal ini. effect `trigger-sit offx.` the. modulatonoscllatorstotransmitaarelatirelyslargef now oir. high frequency; current.tl'1rouglf1=.the7 ultraf violeti'lamprcircuitand cause corresponding ultraf violet' radiation. Toy stabilize` its.l operationA andi increase its: efciencyf thef lamp; 35.*is1N preferably. enclosed; in ar` heat-insulating;` jacketl provided with: a slot-1to1v permit the: passage:-ofrultraeviolet light. throughY registering: slots'E in. theassociatedf scanning'` elements.

The plotting; table' is shown". schematically* in'. F'ig.V 13a-nalindetailin Figs:,3;.4.fand.5=. They-topf! of: the plotting table 'mayl consistoff ai large diam.- eter circular platel of4 glass or? simi1ar-translu cent material 36;. and-'may--be large enough to be Worked on simultaneously byseveralfoperators.. 65

As shown. in Fig; 3, the; topi 36ans supported by cylindricalhousing '31 carriedf onz'a pedestal 38;. Within: the'. housing is arotatable. platform 40 supportedwby4 a-pluralityr of- Wheels; 35. runningy on` a. circular traekfA 4 I?. The; platform 40 carries :two-1 scanningmembers 42 and.4 3which inithefformf. ofthe invention illustratedfareitwofcoaxialv slotted? discs; The2discl42 is provided with a` radiar'slotvv 44; andthe discr4-3is providedwitli aspiralpslot.

ersten-clingil fronrappi'.oxirnatei'yffftheL centerI tofap+- prcniiinately1"thel periph'eryfo the disc 43, arrdli'srotat'eavvitlril the-platform. Theplatformisdriven byftnemot'or 4Gtlirougliameohanioalfdiiferential 4W by; tlfeengagernentl ofI apinioni withkv thevz gear V01 teeth" with" which tl'ie periphery ofi" the platform 4131is*pi'ovi'd't-:d` Supplemental driving meansV is provided@ for" the spirally' slotted disc; 434 in'- the f'orm'lef almotorlwoarried on the platform-4ll`fand`fV gears-connecting motor 4814 Witlf'i the coaxialcentra-li shaft offtlie` disc 43.V The gear ratiosl aresucliithatthelangular rate of "rotation of--spirall'y slotted di'se143i is Big-li as-compared`= Withthat'- of radially sl'ctteddisc' 421 The ultra-violet lamp 3.*'iisca'rried` on a bracket.attachedtothe tubular coaxiali shaft that connects the radiallyY slotted scanning1 dise 422 with the= rotatable 1 platform; 4U. TlieJ-lampi 35 therefore rotatesv Witli` the disefl 421, andiis=sofpositionedlthat it lies directly beneaththe=radially`=extending slot 44 in scanningdisewl'z. Therefresultraf-Violet* light from the lamp i isZ in positionl tofpa'ss tlirouglif any` portion off the lengtlrofth'eradlal' sltll'lwhion'fis not obstructed" bythesuperposedicoaxialscanningsdisc 43T.

Thelspiifallyislbtted disc" 43T2 h'a-'s a ratefl of rota'- tion-such that thel spiralslot`f45 seamsz the radialid'adally slotted disc-ft2- to` malte one` completerevolution. Where theY t'Wo" slots=intersect; suoli: ultra-violet light as is prodlloedi-'in they lamp'.A 35 isfrpermitted tovpassl, asffsliown-infFig; 5, t'o the lowerf' surfaceci, ltli'e transliioent top`= 3B: oiY the plbtting-tabler The under=surface ofthe-translucent-or transs- 4ovx parenttopSBis providedwith alpliosphorescent tand their associatedbrushes-.f

Various expedients" known in thek art but notA illustratedA herein-*may bef employedf for limiting the'eiectivefspreado'the Vlight field of the=u1tra violet radiation from the' lamp 35; andt forv focussingftl'ie'radiation to as` great ani-extent' as possible uponv a small? area off the-plios-phorescent coating 56 immediately above'tlievaperture for-medl by* the intersection off` thetwo scanning slots.y One such expedient is theuseoffaflongcylindrical lens such! asa quartzfrod Wl'ii'ehf mayfbe'- carried inorf'irp close= association W-itlfthe radial" slot' 44? int theA scanning memberf 422A 'Eher rotating: platformL 4W and the' associated radially slotted:scanning'memberll are arranged torbe! drivenl` in synchronism the azimuthy sweep of the'y radar' antenna Fig and? the V spirally slotted;4 scanning.4A member 431i.' is arranged' toloev 45:. Tnespiralslot.4E-consistsfofaisinglaspiral 75y driyenrin: synchronismzwithithe range. scanning.V

rotation of the brush 3| of the cumulator com,-

mutator. The antenna I1 is rotated through speed reduction gearing by the synchronous motor.

i8, and the brush 3| of the cumulator commutator is rotated through speed reduction gearing and mechanical diierential gearing 58 by the synchronous motor 59. The motors 46 and 48 associated with the plotter, and respectively driving the rotatable platform 40 and the spirally slotted scanning disc 43, are synchronous motors; and these, as well as the synchronous motor I8 for effecting the range sweep of the antenna and synchronous motor 59 for electing the range sweep of the cumulator commutator brush 3l, derive their alternating current power from the same source of supply and therefore 'are maintained in operating synchronism with each other. By means of -the mechanical differential 4l `the azimuth phase relation of the radially slotted scanning disc 42 with the map of the exploratory area on the plotting table 36 may be manually adjusted to correspond with the azimuth phasel relation of the antenna Il with the exploratory area; and by means of the mechanical differential 58 the range scanning sweep ofthe cumulator commutator brush 3l is manually adjusted to correspondence with the scanning sweep of the spiral slot in scanning member 43 over the radial slot in scanning member 42. Thus adjusted the target indications on the map of the plotting table each appear at the point in azimuth and range onv the map that corresponds withthe actual location in azimuth and range of the object or target in the exploratory area.

What is claimed is:

1. In anobject location system, a scanning antenna system rotating in azimuth, means fory projecting from said antenna system-a series of energy pulses into sequentially scanned portions of an exploratory area and receiving reflected energy impulses from objects lying in each portion as it is scanned, range scanning means for v receiving and distributing the energy of said re- .iiected impulses in accordance with the elapsed time interval between each projected pulse and each of the reflected impulses, a plotting table having a photoluminescent surface,'an azimuth scanning member juxtaposed and in close contiguity to said table and having an azimuth slot moving in synchronism and phase with the azimuth rotation of said scanning antenna system, a range scanning member in like association with said table and having a range slot moving in synchronism and phase with the movement of` said range scanning means, a source of ultraviolet light, and means controlled by the distributed reflected energy from each of said objects to energize said light source to cause ultraviolet light to pass to said photoluminescent surface at a corresponding point of intersection of said azimuth scanning slot and said range scanning slot, whereby a luminous signal is produced on the photoluminescent surface at a point corresponding in azimuth and range with the location of the object from which the distributed reected energy is received.

2. In an object location system, a scanning antenna system rotating in azimuth, means for projecting from said antenna system a series of energy pulses into sequentially scanned portions of an exploratory area and receiving reected energy impulses from objects lying in each portion as it is scanned, range scanning means for receiving and distributing the energy of said reiected impulses in accordance with the elapsed 8 time interval between each projected pulse and each of the reected impulses, a plotting table, two scanning members, `juxtaposed to said table and in close contiguity thereto, rotating in synchronism and phase, respectively, with the azimuth rotation of the scanning antenna and the range movement of the range scanning means, a source of light also juxtaposed to said table and scanning members, and means controlled by the distributed reflected energy from each of said objects to energize said light source to cause lightv to pass to the surface of said plotting table under the control of said plotting table scanning memx bers, whereby a luminous signal is produced on the surface of the plotting table at a point corresponding in azimuth and range with the location of the object from which the distributed reflected energy is received.

3, In an object location system, a scanning antenna system rotating in azimuth, means for projecting a series of energy pulses into sequentially scanned portions of an exploratory area and receiving reflected energy impulses from objects lying in each portion as it is scanned, range scanning means for receiving and distributing the energy of said rellected impulses in accordance with the elapsed time interval between each projected pulse and'each of the reflected impulses, a plotting table having a photoluminescent surface, scanning members, juxtaposed and in close contiguity to said table and comprising coaxial discs substantially coextensive with said surface, one of said discs having a radial slot and the other a spiral slot centering on the common axis, means for driving said discs in synchronism and phase with said scanning antenna 'and said range scanning meansrespectively', a source of light, and means controlled by the distributed reflected enn ergy from each of said objects to energize said light source to cause light to pass to the surface of said plotting table at a corresponding point of intersection of the radial and spiral slots of said scanning members, whereby a luminous signal is produced on the surface of said plotting table at a point corresponding in azimuth and range with the location of the object from which the distributed reflected energy is received.

4. A large scale automatic plotter, comprising a plotting table, a source of light, modulating means therefor responsive to energy'irnpulses to be plotted, means for directingthe modulated light to different circumferential and radial portions oi said plotting table to produce luminous indications thereon, said means comprising coaxial slotted discs interposed in the path of light to said table from said source, one of said discs containing a radial slot and the other a spiral slot centering on the common axis, and means for differentially driving said discs, whereby said spiral slot scans the parts of the surface of said tablev sequentiallyl exposed to the light through said radial slot, the distribution of the luminous indications over the surface of the table being determined by the time distribution of the light modulating impulses.

` 5. A large scale automatic plotter, comprising a plotting table, a source of light, modulating meanstherefor responsive to energy impulses to be plotted, means for directing the modulated light to different circumferential and radial portions of said plotting table to produce luminous indications thereon, said means comprising rotatable members interposed in the path of light to saidtable from said source, one of said inem-l bers containing .a radial slot and the other intersecting slot arranged to be advanced from one end to the other of said radial slot in each complete revolution of the corresponding member, and means for differentially rotating said members, whereby said intersecting slot admits modulated light from said source to the parts of the surface of said table sequentially exposed through said radial slot, the distribution of the luminous indications over the surface of the table being determined by the time distribution of the light modulating impulses.

6. A large scale automatic plotter, comprising a plotting table having a photoluminescent surface, a source of ultra-Violet light, modulating means therefor responsive to energy impulses to be plotted, means for directing the modulated ultra-violet light to different circumferential and radial portions of said surface to produce luminous indications thereon, said means comprising rotatable members interposed in the path of light to said table from said source, one of said members containing a radial slot having a length substantially that of the radius of said photoluminescent surface and the other an intersecting slot arranged to be advanced from one end to the other of said radial slot in each revolution of the intersecting slot member, and means for rotating said members at diierent speeds such that the intersecting slot admits ultra-violet light from said source sequentially to the photoluminescent surfaces of the table from one end to the other of said radial slot for each small angular advance of said radial slot member, the radial and circumferential distribution of said luminous indications over the surface of the table being determined by the time distribution of said energy impulses.

'7. A large scale automatic plotter, comprising a plotting table, a source of light, modulating means therefor responsive to energy impulses to be plotted, means for directing the modulated light to different circumferential and radial portions of said table to produce luminous indications thereon, said means comprising rotatable members interposed in the path of light to said table from said source, one of said members having a radial slot and the other an intersecting slot arranged to be advanced from one end to the other of said radial slot in each complete revolution of the intersecting slot member to admit light to the corresponding sector of said plotting table, a common rotatable support carrying both said members, driving means for causing the rotation of said support, and supplemental driving` ing the same in accordance with the signals to be plotted, and means controlled by the positions of said scanning members with respect to each other and the table to distribute light from said source to different circumferential and radial points on said table in accordance with the time distribution of the signal modulations.

9. A large scale automatic plotter, comprising a plotting table having a photoluminescent surface, a support rotatable With respect to said table on an axis coinciding with the center of the table, said support carrying a member having a radial slot adapted in the rotation of said support to scan said table circumferentially, a member having a spiral slot also carried by said support adapted to scan radially the portion of said table that is being scanned circumferentially by said radially slotted member, a source of light, means for modulating the same in accordance with signals to be plotted, and means controlled by the positions of said scanning members With respect to each other and the table to distribute light from said source to different circumferential and radial points on the photoluminescent surface of said table in accordance with the time distribution of the signal modulations.

10. A large scale automatic plotter, comprising a plotting table having a photoluminescent surface, two scanning elements rotatable in a plane in close proximity to and substantially coextensive With the plane of said photoluminescent surface, one of said elements having a radially eX- tending slot and the other an intersecting slot arranged to be advanced from one end to the other of said radial slot in each revolution of the intersecting slot member, a source of signalmodulated light, and means for rotating both said members With respect to the table and at diierent rates with respect to each other such that said intersecting slot admits signal-modulated light from said source sequentially to the photoluminescent surface of the table from one end to the other of said radial slot for each small angular advance of said radial slot member and in accordance With the time distribution of the signal modulations.

RICHARD W. BENFER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES- PATENTS Number Name Date y 1,418,240 Curie May 30, 1922 1,810,610 Jones, Jr June 16, 1931 1,924,174 Wolf Aug. 29, 1933 2,063,906 Boch Dec. 15, 1936 2,363,600 Lawlor Nov. 28, 1944 2,426,245 Skellett Aug. 26, 1947 FOREIGN PATENTS Number Country Date 520,778 Great Britain May 3, 1940 542,634 Great Britain Jan. 21, 1942 

